In cellular radio systems a User Equipment (UE) can move over a geographical area covered by the system. The overall geographical area is covered by many small areas typically denoted sectors. A sector implies the geographical area covered by a radio unit and is typically also the geographical area of a cell. When the user equipment moves from one cell to another the connection between the system and the user equipment must be handed over from the cell where the user equipment is currently camping to the new cell. This is often referred to as a handover.
In existing cellular radio systems such as a Wideband Code Division Multiple Access (WCDMA) radio system a User Equipment (UE) receives a downlink (DL) signal from a first (remote) radio unit ((R)RU) and can perform a so called softer handover to another second Radio unit by simultaneously being connected to both the first and second radio units. All cells are connected to a Radio Network Controller (RNC) via an RBS. The RNC controls all handovers. An exemplary setup illustrating such a scenario is depicted in FIG. 1. FIG. 1 depicts a radio network controller (RNC) connected to a radio base station (RBS). The radio base station comprises a main unit and a number of radio units (RRU A, RRU B and RRU C) connected to the main unit. The different radio units are related to different cells (Cell A, Cell B and Cell C). In this example the radio units are remote radio units deployed at a distance from the main unit at different transmission sites. One or many of the radio units can of course also be co-located with the main unit. The radio units are connected to the RNC via the main unit by cables such as optical cables.
The difference between the distances to the UE from different antennas for the different cells must be less than the narrow search window if the softer handover shall work.
In the set-up being depicted in FIG. 1, every sector is regarded as one cell and every (R)RU has its own scrambling code.
A problem with existing systems is that an increased number of radio units which the RNC need to control may strain the limited resources of the RNC. Also, in areas with many (R)RUs, there is a problem with interference between neighbouring RUs.